Single Shot Temporal Characterization of SASE XUV FEL Pulses

Summary form only given. Since the free-electron laser FLASH at DESY in Hamburg, Germany [1] lases in Self-Amplified Spontaneous Emission (SASE) mode each pulse is "unique" and has different pulse energy, XUV spectrum and pulse duration. Due to very small fluctuations in the acceleration, the arrival time of the XUV pulses jitters within several tens of femtoseconds. A Terahertz (THz) field driven streak camera [2,3] has the potential to deliver single-shot pulse duration information basically wavelength independent and with a high dynamic range (in pulse duration and FEL energy) and it is able to be operated with repetition rates up to several hundred kHz (potentially even MHz). In addition, it can provide arrival time information between the XUV pulse and the laser driving the THz generation for each single pulse with accuracy well below 10 fs resolution. The measurement principle is based on a noble gas target being photo-ionized by the FEL pulse. The kinetic energy of the resulting electrons is modified by the electric field of the THz radiation, when it is co -propagating through the target. If the electron wave packet is short compared to the period length of the terahertz fi eld (> 500 fs in our case), the temporal structure of the wave packet will be mapped onto the kinetic energy distribution of the emitted electrons [4]. The pulse duration can be extracted from the broadening of the peaks measured in the photoelectron spectrum due to the presence of the THz fi eld. The shift of the kinetic energy peaks provides the arrival time. The THz streak camera was build and installed at FLASH at the PGO beamline. This beamline has the capability to use the zero order FEL beam for the streaking set up while the dispersed radiation can be simultaneously used in the PG2 hemline to measure the FEL spectrum with high resolution. The THz radiation is generated by interaction of pulses delivered from the FLASH1 pump-probe laser (800 nm, 80 fs, 6.5 mJ) with a nonlinearly reacting crystal. In detail, the source is based on pulse front tilt optical rectification in a Lithium niobate (LiNb0 3 ) crystal [5]. A THz field strength of -300kV/cm has been achieved in the interaction point.